Mine is a 34H so things are different. But our intake is on the floor in the bedroom feeding the heater direct, and TWO larger vents located on the wall in the bedroom and another in the hall, both feeding to the heater through a single return hole into the heater located in the wall behind one of the grilles. The problem I found on two different 34H's, two different years, one Journey, one a Meridian, is the floor cutout, similar to the picture above, had a thick piece of vapor barrier plastic over it, meaning more than 50% of the designed return vent space was completely blocked... They never cut the floor vapor barrier plastic covering this return hole from the factory.. So if you have two or more intakes, make sure all returns are wide open. Especially those reading this with a 34H.
(Edit: added words)